Project Summary The acute respiratory distress syndrome (ARDS) affects more than 190,000 patients each year in the US and continues to have a 30-40% mortality rate. Our research group recently reported that levels of cell-free hemoglobin (CFH) are elevated in pulmonary edema fluid from the distal airspaces of patients with ARDS and that increased airspace CFH is associated with increased alveolar-capillary permeability, suggesting that CFH may contribute to ARDS pathogenesis and may be a new target for ARDS therapy. The goals of this proposal are to advance our knowledge of the cellular and molecular consequences of CFH in the airspace and test a CFH-targeted therapy in the airspace. Our preliminary data establish a unique model system that we will harness to define the proximal regulators of acute inflammation in the lung. Instillation of purified endotoxin- free CFH into the airspace of mice is sufficient to induce robust alveolar inflammation and has identified alveolar macrophages as a key target of CFH. Furthermore, mice deficient in TLR4 have attenuated inflammation in response to CFH, identifying one key pathway in its mechanism. We also have determined that CFH is engulfed by only a subpopulation of macrophages in the airspace and that these CFH-containing macrophages have less pro-inflammatory cytokine production than cells lacking CFH. Based on these data, we propose that intra-alveolar CFH may modulate both pro- and anti-inflammatory pathways during acute lung injury and that the balance between these pathways could be manipulated in the airspace as a novel therapy for ARDS. In this proposal, we will test the hypothesis that CFH causes airspace inflammation through TLR4- dependent NF-?B activation in alveolar macrophages and that augmentation of CFH uptake into macrophages with haptoglobin will ameliorate acute lung injury through attenuation of macrophage-dependent inflammation. In Aim 1, we will use transgenic mice with macrophage-specific TLR4 deletion or NF-?B inhibition to determine the role of macrophage TLR4 and NF-?B activation in CFH-mediated airspace inflammation. In Aim 2, we will use CD163 null mice and a novel application of magnetic cell sorting to determine whether uptake of CFH into alveolar macrophages alters macrophage polarization towards an anti-inflammatory state. In Aim 3, we will test intra-alveolar haptoglobin as a new therapy for ARDS using a mouse model of bacterial pneumonia and will determine the clinical importance of haptoglobin in the airspace during ARDS. By the completion of these studies, we will have identified the cell-specific and molecular mechanisms of a novel mediator of ARDS and tested a new localized ARDS therapy that has high potential for rapid translation into human clinical trials. In addition, through these studies, a promising young physician scientist will gain new skills in basic and translational studies of ARDS under the guidance of a highly accomplished mentorship committee of experts in ARDS, inflammation, and macrophage biology. These new mentored skills will form the foundation for this junior investigator to achieve a long-term goal to be an independently funded academic physician scientist.